U.S. Pat. No. 4,778,596 provides semipermeable membranes of a porous polymeric substrate whose external and all internal surfaces (pores) are encapsulated by a semipermeable thin film of a hydrophilic polymer. The encapsulating film consists of at least one layer of said hydrophilic polymer which is cross-linked/stabilized by polyfunctional compounds containing at least two functional (reactive) groups. Such membranes show improved separation behavior as well as enhanced performance lifetime at relatively high and low pH, temperature and pressure. The pH range within which it is possible to use such membranes effectively, was described as being from 2 to 12.
Because many common and important processes involve process, product and waste stream involving greater extremes of pH, the advantages of providing a process by which membranes that can operate effectively in the pH range from 0 to 14, are self evident.